1. Field of the Invention
This invention pertains to a method and apparatus for thermally developing a photosensitive element, and particularly to a method and apparatus for supporting a photosensitive element during thermal development.
2. Description of Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions typically include an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, polymerization of the photopolymerizable layer occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. Conventionally, the element is treated with a suitable solution, e.g., solvent or aqueous-based washout, to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. However, developing systems that treat the element with a solution are time consuming since drying for an extended period (0.5 to 24 hours) is necessary to remove absorbed developer solution.
As an alternative to solution development, a “dry” thermal development process may be used which removes the unexposed areas without the subsequent time-consuming drying step. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. No. 3,060,023 (Burg et al.); U.S. Pat. No. 3,264,103 (Cohen et al.);U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens); and U.S. Pat. No. 5,279,697 (Peterson et al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is removed from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the flowable composition from the unirradiated areas and form a relief structure suitable for printing. After such processing, there remains a raised relief structure of irradiated, hardened composition that represents the irradiated image.
Processors for thermal development of flexographic printing elements are known. U.S. Pat. No. 5,279,697 describes an automated process and apparatus for handling an irradiated printing element and accomplishing repeated heating and pressing to remove the unirradiated composition from the element. The apparatus includes a preheating drum that has a clamp flush mounted transversely on an outer surface of the drum for securing a leading edge of the photosensitive element to the outer surface of the drum. The preheating drum is coated with a silicone rubber composition layer having a Shore A hardness rating between about 30 and 60. The rubber coating may be impregnated with aluminum particles.
WO 2001/18604 also describes a method and apparatus for thermal processing a photosensitive element. The apparatus includes a drum coated with a rubber composition or a thin tackification coating on a smooth metallic surface of the drum. The rubber coating provides a resilient surface that results in a longer nip zone as a result of the surface deflecting under the pressure exerted by a developing hot roller. The tackification coating temporarily adheres the photosensitive sheet element to the drum, and since no rubber layer is present, can improve the thermal conductivity between the drum and the photosensitive element.
In an embodiment of a thermal processor, commercially identified as CYREL® FAST 1000TD processor, an exterior surface of the drum includes a silicone rubber layer having a Shore A hardness of 50, and a tackification layer on the rubber layer. The tackification layer is DOW 236, a silicone dispersion in solvent, available from Dow Corning.
During thermal development, the photosensitive element is mounted on the drum with the element residing on the tacky layer. Unfortunately with use, the layer or layers on the surface of the drum can deteriorate and detrimentally impact the performance of the system. With use, the tackiness of the tacky coated surface decreases due to dirt collection and wear of the coating. The tacky layer can also be abraded away by the action of the absorbent material on a hot developer roll and the photosensitive element traversing through the nip. In some cases, the tacky layer is abraded to the underlying rubber layer. Insufficient tackiness of the tacky layer will not retain the photosensitive element, particularly a trailing end of the element, in place as the drum rotates the element through the developing cycle of heating the element and contacting with the absorbent material. If the photosensitive element is not appreciably held in contact with the drum, the element can rub against other parts of the processor and damage the relatively soft printing surface. Depending on the orientation of heating elements, the photosensitive element can possibly contact hot surfaces of the heating elements, resulting in damage to the photosensitive element, or the heating elements, or both.
The surface layer or layers can also crack or be cut by knives or other tools that are required on occasion. The photosensitive element can be damaged by large cracks or crevices in the surface layer since the element can be forced into the crevice due to the pressure exerted at the nip during development. The integrity of the drum coating can also be compromised. Smaller cracks can tend to grow into larger cracks due to the forces inherent in the nip.
In the event of any of the above problems with the surface layers on the drum, the entire drum must be replaced. Replacing the drum is expensive as well as a cumbersome process that requires lifting equipment to remove the existing drum from the apparatus. Additionally, the apparatus is not operational for several hours to a few days, resulting in downtime for the customer.
So a problem arises with the quality of the layer or layers on the surface of a development drum used in thermal development of photosensitive elements. With use, the layer or layers on the drum can become worn and/or damaged which can impact the thermal development system performance.